Extract time information in `from_csv` function

[rogerkuou]

Following the discussion in #39 , the from_csv function should extract time coordinates by assuming a time string pattern in the header of space-time columns. The following default behavior should be applied:

1. Assume the string pattern yyyymmdd for time information
2. Assume the same pattern is shared by all space-time columns
3. The time coordinates should be stored in np.datetime64 format which is adapted by Xarray.